Wearable Board Standard Idea

First Draft: Jan 9th, 2000 Revision: 0.0.4, Feb 4th, 2000 (This document is a work in progress.) (Feel frees to send comments and suggestions to Bryan Andersen.)

Recent changes:

0.0.4: Added Open License statement.
0.0.3: Added pinout possibilities for PCI bus between boards.

This Wearable Board Standard (WBS) is open. I welcome others inputs to help come up with a final standard. I would even welcome the inclusion of work by a standards body. Anybody or any corporation may use it without licensing fees under one condition: Any extensions or modifications made shal also be made open and usable by anybody or any corporation.

Something that has crossed my mind is how to allow for expanding functionality in a wearable computer. Currently we use PC/104 or PC/104-Plus. These both have the ruggedness a wearable needs, but they don't have the compactness. They are close, but not quite there yet. I think it's necessary to get the wearable down below 140mm tall by 90mm wide by 25mm deep plus an equal sized battery pack. One about twice the width could be produced to provide more capabilities and storage. One of the goals I have for doing this is to provide for a bus that is capible of having full broadcast quality video pushed over it while still having enough free time for other uses. If only compressed DV video is going over the bus it is possible to do on just the standard PCI bus, but when HDTV digital cameras become common I don't expect there to be to much free time on the bus. I'm aiming for PCI-X as this gives us an effective 4x speed increase, and eletrically it is specified. Another reason for aiming higher is the existance of Small-PCI and Mini-PCI. Both new standards designed for the notebook market. Both Small-PCI and Mini-PCI have form factor limitations. While I specify a board size, I only specify a minimum board spacing of 3 mm with 0.5mm of that going to interboard gap. With redesign of the Hirose connectors one should be able to allow for different stacking heights. Of the 3mm I give 1.5mm to the lower board, and 1 to the upper board in a stack. Any additional height a board needs must be accompanied by additional height on the stacking connector they provide. So if a board wants to have 3mm of components on it bottom side it must provide an additional 2 mm of stacking height in it's botom side connectors. If it wants 7 mm in it's top side it must provide an additional 5.5mm to it's stacking height on the top.

I did a quick look through the DigiKey catalog and found a possibly suitable connector by Hirose Electric Co. It's the FX8 .6mm connector. Are there other connectors in a similar size range that would also be suitable, they need to be explored. I expect the connector needs to be at most 8mm wide and 50mm long in it's overall dimensions. The FX8 connector comes in 60 to 140 pin versions for 25 to 49mm overall connector length respectively and about 8mm width for the board pads. It is a surface mount connector. The PCI bus could be done over one of the 120 or 140 pin ones. Using the 120 pin connector means 4 pins would need to be droped from the PCI pinout spec, but this is possible because of the keying slots used on the standard PCI connector. Going with the 140 pin one allows for a greater number of ground and power pins, or adding specific IO type pins. Two connectors would need to be used to provide a 64bit PCI bus. They have a 3mm board to board stacking height, and they look rugged enough for wearable use. The board stacking height seams a little shallow to me, I'd prefer 5-10mm, but they could be redesigned for the added height or different boards could have different heights. One could supplement them mechanically with hex board spacers like used on PC/104 card stacks to get added strength and vibration resistance needed in a wearable. Some redesign would be needed to allow for stacking more than two boards. The connectors have guide pins that would need to be redesigned to allow connectors to be placed on both sides of a PCB in the same spot. For now this can be dealt with by just grinding them off.

I was thinking the main CPU card could be placed on the "bottom" of the stack so the CPU can be hard butted up against the case to get heat sinking. Alternately cross flow ventilation could be provided by micro fans.

For board size I was looking at 70mm by 100mm which is the format of a laptop HD. The PCI connector would be placed at one end. IO connectors would be placed at the other end. If a 10mm wide space is planned for in the case at the IO end one would have reasonable room for cabling to hook up to boards and run to external connector blocks. A standard IO and Power connectors could be designed so cases could be standardized. I see an IO block being set to board stacking height + PCB thickness, 70mm long + 5mm mounting flange on each end, and 15mm deep. Each one provides external connections for the board it supports. The reason I separate the IO block from the board is so it can move with the wires and not cause the board to flex and die. I see flat flexible cable being used to hook the IO block to the PCB. To allow for variable thickness in PCB and stacking heights the mounting flange for the IO blocks doesn't have screws. The blocks instead go into a slot that tightens down to lock them in place.

Cases could be designed for either forced air cooling, or radiative cooling. The problem with radiative/convective cooling of interior cards is it may not be enough. There are ways around this, possibly a fan to circulate air in the case only, but I'm not a thermal engineer.

Reasons for choosing sizes:

70mm wide is wide enough to allow a PCMCIA card to fit between the board stacking posts.
The variable face height size of the IO block allows one to have a bezel to allow for one or more PCMCIA cards to be handled in a stack. It also allows a card that needs allot of IO to have the needed space. Board density suffers though, but that is life.

Possibilities:

One could allow double width mother boards. You would have two card stacks plus a gap between them. Likely 5mm or so. This is to allow for holding down the IO Blocks. This is a way to provide more base functionality while keeping the overall thickness of a system reasonable. This would give a double width cpu board a size of 145mm by 100 mm and give it two IO blocks. In this light double width expansion cards could also be done, but they would need to be at the base of the stack. Most likely relegated to Video.
One could have HD drives built to the standard that connect in directly to the PCI bus and mount into the stack.
AGP style handling of video or would it be best to go to 66Mhz/64bit PCI bus (PCI-X)? What happens when you place a 33Mhz board on a 66Mhz PCI bus? How does the standard handle this? Some research needs to be done. (www.pcisig.com). There appears to be a Mini PCI standard in the works for doughtier cards in portables. News releases say it's about 2" by 3" and 1/4" think. This meets our design size needs for a wearable. The news releases also say it's been primairly designed for Moden and LAN use, and has special IO pins for carrying signals for such. Would it do for wearable computers?

PCI Stack Mechanical Specifications

Mounting holes are at the corners of a rectangle 90mm long by 62mm wide. This rectangle's center is coincident with the center of the rectangle that forms the board edge. This means the mounting holes are 4mm in from the corners on the width, and 5mm in from the edge on the long length.
Mounting holes are for M3 screws and hex posts.
All the mounting posts should be isolated from electrical ground. They will be connected to case ground.
The PCI Stack connector's center line goes through the centers of the mounting holes on its end of the PCB. Its length is centered between the mounting holes.

ASCII Art Time

Single width PCI Stack board

    |------------------100mm-----------------|
    | |---------------90mm---------------| |
5mm-|--|                                  |--|-5mm

    +----------------------------------------+ ------
    |                                        | |4mm |
    | o                                  o | ---- |
    |                                        |    | |
    | [X] < pin one at this end              |    | |
    | [X]                                    |    | |
    | [X]                                    |    6 7
    | [X]                                    |    2 0
    | [X]    Top of board                    |    m m
    | [X]                                    |    m m
    | [X]                                    |    | |
    | [X]                                    |    | |
    |                                        |    | |
    | o                                  o | ---- |
    |                                        | |4mm |
    +----------------------------------------+ ------

o = mounting hole
[X] = PCI Stack connector

Double width PCI Stack board

    |------------------100mm-----------------|
    | |---------------90mm---------------| |
5mm-|--|                                  |--|-5mm

    +----------------------------------------+ -------
    |                                        | |4mm |
    | o                                  o | ---- |
    |                                        |    | |
    | [X] < pin one at this end              |    | |
    | [X]                                    |    | |
    | [X]                                    |    6 |
    | [X]                                    |    2 |
    | [X]    Top of board                    |    m |
    | [X]                                    |    m |
    | [X]                                    |    | |
    | [X]                                    |    | |
    |                                        |    | |
    | o                                  o | ---- 1
    |                                        | |    4
    |                                        | 17mm 5
    |                                        | |    m
    | o                                  o | ---- m
    |                                        |    | |
    | [X] < pin one at this end              |    | |
    | [X]                                    |    | |
    | [X]                                    |    6 |
    | [X]                                    |    2 |
    | [X]                                    |    m |
    | [X]                                    |    m |
    | [X]                                    |    | |
    | [X]                                    |    | |
    |                                        |    | |
    | o                                  o | ---- |
    |                                        | |4mm |
    +----------------------------------------+ --------

o = mounting hole
[X] = PCI Stack connector

Notes:

I was debating having a set of holes that matched the bottom mounting hols of 2.5" Laptop hard disks. I then decided that it was better to mount the HD to the case and not try to mount it into the stack.

PCI Stack Connector Pinout

WBS Standard PCI possibility one using a 120 pin Hirose connector. This will provide specs equivalent to the current PCI spec used in PCs with the exception of voltage levels. 3.3VDC IO is prefered over 5VDC IO for lowering power use. I'm afraid that power loading per pin will cause the need to go to the 140 pin connector. I need to investigate this.
WBS PCI-X possibility using two connectors, and allowing for 64bit/66MHz transfer rate. This would be ideal for video wearables. I need a copy of the PCI-X standard to be able to do much more with this possibility.

PCI Stack Electrical

To Be Determined
It is expected that the allowable electrical load per card will be significantly lower than regular PCI. This is due in part to the .4A load allowance of the contacts in the connector.

Parameter	Single Width Board	Double Width Board	Description
MotherBoard
Width	70mm	145mm
Length	100mm	100mm
Inter Board Gap	3mm	3mm	On the side opposite the PCI Stack height is whatever the manufacturer wants. On the PCI Stack side components are limited to 2mm height unless additional height is provided by increasing the height of the connectors. On double width boards that provide two PCI Stacks, both stacks must have the same stacking height. This is to allow multiple double width boards to be stacked. Note: a double width motherboard may not decide to provide a second PCI Stack in the expectation that the space would be used by peripherals.
PCI Stack Board
Width	70mm	145mm
Length	100mm	100mm
Inter Board Gap	3mm	3mm	How far between the circuit boards. This space is the minimum allowed mounting height. It is expected that boards will have components on both sides. To allow this and to allow for reliable stacking ability. A board can have components that are up to 0.5mm high on the motherboard side, and 2mm on the opposite side unless they provide for greater depth by using higher connectors on one or both sides. This gives a .5mm gap between boards for air flow and vibration flexing. If greater air flow is needed, the the board should provide for it by increasing the height of it's stacking connectors. On double width boards that provide two PCI Stacks, both stacks must have the same stacking height. This is to allow multiple double width boards to be stacked.
IO Block
Width	80mm	80mm	70mm wide useable are with two 5mm wide mounting flanges on each end.
Depth	15mm	15mm	This is how deep connectors can protrude in from the outside face of the face plate.
Height	3mm+	3mm+	The actual height is determined by the height that the board uses in the PCI stack. So if a board uses the default stacking heights for the top and bottom sides it will be 3mm plus the thickness of the PCB.

Some ASCII art. ( I need to draw this up with a proper CAD package. Suggestions for Linux open source one?

                                         .-IO Block Zone
                                        \/
+-----------------------------------------+<- Case inside
|# CPU ETC.                     #   XXXXXX|
|================================   XXXXXX| <- CPU board
|#PCI AGP                       #         |
|================================         | <- Video board
|#PCI                           #         |
|================================         | <- Whatever board
|#PCI                           #         |
|                                         |
+-----------------------------------------+

View from side of card stack

=           PCB edge
|, +, -        case interiors
#           screw posts
XXXXXX      IO Block
XXXXXX

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